In conventional fuel injection systems, the fuel injectors may be mechanically, hydraulically, or electrically actuated. In mechanically-actuated systems, the pumping assembly which periodically causes fuel to be injected into the engine cylinders is mechanically coupled or linked to a cam driven by the engine so that the pumping assembly is actuated in synchronism with the rotation of the cam. In hydraulically-actuated systems, the pumping assembly is hydraulically driven by pressurized actuating fluid which is selectively communicated to the pumping assembly by an electronically-controlled valve. One example of a hydraulically-actuated, electronically controlled fuel injection system is disclosed in U.S. Pat. No. 5,121,730 to Ausman, et al.
In conventional fuel injection systems which incorporate a poppet valve with one or more frusto-conical seats, the poppet valve may bounce after it initially hits its mating seat member in the process of closing. Since a tight seal will not be formed between the poppet valve and its seat until the bouncing stops, and since the bounce is typically not consistent for successive injections, the time at which fuel injection starts may vary undesirably injection to injection. Although this problem may be overcome with hydraulic damping systems or spring over-travel mechanisms, these solutions are relatively complicated and/or expensive.
Spool valve assemblies are typically formed of a generally cylindrical bore formed in a valve housing and a generally cylindrical valve element disposed for reciprocating movement within the cylindrical bore. The diameter of the each of the two ends of the cylindrical valve element is typically greater than that of the middle portion of the valve element. A plurality of fluid passages or bores are formed in the valve housing, and the reciprocation of the valve element selectively opens, closes, and/or interconnects the fluid passages. Examples of spool valves are disclosed in U.S. Pat. No. 5,042,445 to Peters, et al.